748 MODE OF SPREAD OF INFECTIVE DISEASES. 



resistant than the infective agents of the acute exanthe- 

 mata, and although both are produced in large quantities 

 and distributed in the surrounding world by the patient, 

 the latter are, nevertheless, incomparably more contagious, 

 because the unprotected surfaces of the body can be 

 readily invaded by them, and these surfaces are very 

 easily infected by contact or by currents of air ; on the 

 other hand, in the case of typhoid fever we usually have 

 to do with a limited mode of transport by means of im- 

 perfectly prepared food and of impure water, and the 

 seats of invasion are difficult of attack and provided with 

 a protective arrangement ; in this case also currents of 

 air can only exercise an influence as a means of trans- 

 port in so far as they curry the specific infective agents, 

 in the first place to the food, and under favourable con- 

 ditions by means of the food to the intestine. 



In the case of some infective agents which are as yet 

 but little known (malaria, relapsing fever, &c.), we can 

 scarcely put forward any hypotheses as to whether 

 specific points of invasion are present at all, or where 

 they should be looked for. 



4. Individual Predisposition. 



Unequal pre- Experience has long ago taught us that the different 

 different 011 f infective agents are not equally dangerous to all kinds 

 species and of o f warm-blooded animals, but that the one can affect only 



different m- . 



HvMnais. this, the other only that species. Closely allied species 

 and races of animals often show in this respect the 

 most striking differences ; thus the bacillus murisepti- 

 cus kills without exception every house-mouse inoculated 

 even with the most minute quantity, while field-mice 

 are not at all affected even by large doses ; and the 

 micrococcus tetragenus is only infective for the white 

 variety of house-mice, while it is not infective for the 

 grey variety. But even among the individuals of one 

 and the same species similar differences exist ; and the 

 infective organisms which are peculiar to man attack for 

 the most part only a certain number of predisposed in- 



